Filtration to Protect Chiller from Fibrous Debris
BACKGROUND
Cooling towers dissipate both ambient and process heat in most large manufacturing
facilities. These structures facilitate the transfer of unwanted energy (heat) from a
transport liquid (usually water) to the atmosphere. Problems with efficient heat transfer,
equipment protection and pathological risks to employees can most often be traced back
to an issue with suspended solids. These solids can originate in the process, in the piping,
from the atmosphere or from internal biological growth. Side-stream filtration is the
most commonly used method of maintaining minimal suspended solids in a cooling
system. Side-stream filtration takes a portion of the flow from the system and filters it to
remove suspended solids and then returns the clean water back to the system usually
through the cooling tower reservoir or sump. This method maintains general control of
suspended solids. It does not filter all the water going to the process. Historically,
cooling systems have relied heavily on the two following established methods of
suspended solids removal. Cyclonic devices are highly efficient at removing high
specific gravity solids, while granular media filtration generally is more effective at
removing low specific gravity organic solids. Automatic self-cleaning screen filter
technology provides a solid barrier to both organic and inorganic solids regardless of
specific gravity and requires very little energy to operate. In addition they conserve
coolant additives by using very little coolant liquid for the self-cleaning process. The net
loss of coolant can be completely eliminated by incorporating the cleaning cycle into the
blow-down process of the cooling system.
APPLICATION
A large producer of cigarettes and fine cut tobacco processes, packages and ships billions
of cigarettes per year. The plant installed a new production line recently with all new
equipment. One new piece of equipment was a state-of-the-art 150 ton magnetic chiller
on the evaporator loop of the cooling system. Chillers are expensive components in a
cooling system that must be supplied a continuous flow of clean water from a cooling
tower. Fibrous solids captured by the cooling tower were particularly problematic for the
equipment. Therefore, it was determined by plant engineers that sidestream filtration was
not sufficient and that all the cooling water going to the chiller would be filtered to
protect it from the possibility of damage induced by suspended solids. Cyclonic
separators could remove the heavier solids such as sand and pipe scale but not the lighter
organic fibers and wind-blown materials. Granular media filters could remove the
fibrous organic fraction of suspended solids but would have trouble with sand particles
since they would tend to stay in the filter vessel during the cleaning cycle. Also, these
filters consume appreciable amounts of energy, need to be taken offline during each
cleaning cycle and require copious amounts of water for backwashing.
SOLUTION
A popular manufacturer of filtration equipment was asked to recommend a filtration
system to protect the chiller from airborne debris scrubbed from the atmosphere by the
cooling tower and other solids originating within the manufacturing process. The
solution had to take into account that the system was likely to contain both organic and
inorganic solids, many fibrous, and that a continuous flow of water must be delivered to
the chiller at all times. Pressure losses had to be kept to a minimum and flow rates could
not vary appreciably. A fully automatic self-cleaning filter with 6-inch inlet and outlet
flanges and a 200-micron stainless steel screen element was recommended for the
application. The filter has a built in bypass system that will automatically open a bypass
line around the filter element should anything cause the filter to fault. A filtration degree
of 200-micron was determined to be most appropriate for this application to remove
particles capable of causing damage to the new chiller. This means the openings in the
screen are a little larger than the diameter of a human hair and will remove airborne
particulates, microbiological growth, cottonwood seeds and other materials found in most
cooling towers. Figure 1 shows the final installation with the built-in bypass system.
OPERATION
The filtration unit consists of two stages of filtration. The components of a typical filter
of this type are depicted in Figure 2. The first is a coarse cylindrical pre-filter (1) to
remove rigid particles that are too large to pass through the nozzles of the dirt collector
during the cleaning cycle. The second stage is a cylindrical stainless steel weave-wire
fine screen element (2) that is the real work-horse of the filter. The openings of this
element are of a specific size representing the filtration degree of the element (200
microns in this application).
Dirty water enters the inlet flange (3) and then passes through the coarse screen described
above from outside in and then passes into the center of the fine screen element. Water
passes through this fine screen from the inside out and exits through the outlet flange (4).
Unwanted solids accumulate on the inner surface of the fine screen element. As this
debris layer builds, energy is dissipated causing a pressure differential across the screen.
When the control system senses this differential pressure reaching a pre-set threshold (7
psi or 0.5 bars in this case) the cleaning cycle is initiated. The first step in the cleaning
cycle involves the automatic opening of the rinse valves (5) to atmosphere. A pipe
carries this flush water to a drain but oversized piping is used to prevent backpressure
against these rinse valves. When these valves open the pressure in the hydraulic motor
chamber (6) and the dirt collector (7) drops abruptly. The pressure just inside the nozzle
openings on the dirt collector is nearly zero gauge pressure since these nozzles are
connected through the dirt collector, hydraulic motor chamber and rinse valves to the
atmosphere outside the pressurized filter body. Water inside the filter rushes into the
nozzles at very high velocity and passes through the dirt collector into the hydraulic
motor chamber and out the rinse valves. The nozzle openings on the dirt collector are
within a few millimeters of the fine screen surface causing water to pass backward
through the screen in a very small area at very high velocity dislodging the filter cake
(debris built up on the inside screen surface) and sucking it into the dirt collector. Since
only a very small area of the screen is being cleaned by each nozzle (an area about the
size of a dime), there is plenty of energy available for vacuuming debris from the screen.
This debris is discharged along with a small amount of water through the rinse valves to
the drain. As water rushes out of the dirt collector into the hydraulic motor chamber it
passes through the hydraulic motor (8) imparting a rotation to the dirt collector and thus
moving the cleaning nozzles around the inside surface of the screen. A hydraulic piston
(9) then slowly moves the dirt collector linearly giving the rotating nozzles a spiral
movement in such a way that every square inch of screen surface is passed by a suction
nozzle assuring that the entire filter cake is vacuumed from the screen during the cleaning
cycle. This entire process takes less than 10 seconds and does not interrupt the flow of
clean water downstream.
SUMMARY
Fully automatic self-cleaning screen filters provide an economical means of removing
suspended solids from cooling tower water. The use of weave-wire screens as the
filtering media provides a positive barrier eliminating all particles larger than the
filtration degree of the screen from the cooling system and many smaller particles due to
the filtration effect of the filter cake that builds on the screen element surface between
cleaning cycles. This phenomenon of filtration improvement can be loosely quantified as
removing particles down to about one tenth (1/10) the size of the screen filtration degree
when the filter cake is at its thickest. This 1:10 relationship is called the capture ratio as
employed in screen filtration systems. The efficient suction cleaning principle allows the
filter cake to be removed completely from the screen surface within seconds without
physically touching the cake or screen. During the suction cleaning cycle the filtration
process is uninterrupted; thereby, providing filtered water downstream of the filter at all
times eliminating the need for redundant equipment. Water and chemical losses are kept
to a minimum and organic and inorganic solids are removed with equal efficiency. Since
only a small pressure differential occurs across the screen element, the extrusion of soft
organic material through the screen is prevented. If any problem should occur with the
filter, the controller will sense this and open the built-in bypass valve to provide a
continuous flow of water to the new chiller. The controller will then send a signal to
notify personnel of the problem for resolution. Routine maintenance is minimal
consisting of a monthly inspection of the rinse valves to see that they are seating properly
and an annual inspection of the screen and hydraulic piston. An occasional manuallyinduced cleaning cycle by maintenance personnel is recommended to assure proper
operation. Full stream protection, automatic self-cleaning process, automated bypass
system and low maintenance were just the qualities the engineers were looking for in a
protection system for the new magnetic chiller.
Automatic Bypass Valve
Figure 1. Filter Installation
Figure 2. Filter Components
Author:
Dr. Allhands received his engineering BS from Purdue University and his
PhD from the University of Florida. He is a registered professional engineer in Illinois
and Florida. Marcus spent seven years as Water Quality Manager of an engineering and
environmental consulting firm in southwest Florida and then ten years as Senior
Application Engineer for an international filtration company. Marcus has held the
position Vice President of Business Development since 2006 for Orival, Inc., a broad
spectrum filtration company.
Marcus N. Allhands, PhD, PE
Vice President of Business Development
Orival, Inc.
(765) 987-7843 office
(551) 206-8526 cellular
ma@orival.com
www.orival.com
Company:
Orival, Inc. has been manufacturing and shipping automatic self-cleaning
filtration systems world-wide for over twenty-six years. Orival has headquarters in
Englewood, NJ and maintains a complete inventory of carbon steel and stainless steel
filters in sizes from ¾” to 24” and filtration degrees down to 10 microns. Orival also
manufactures manifolds for multiple filter units and complete skid mounted and container
filtration systems. Visit www.orival.com or call (800) 567-9767 for more information.